In recent years, multimedia telephone terminals which can be connected to fixed networks have been developed. These terminals provide real-time video, audio, or data, or any combination thereof, between two multimedia telephone terminals over a voice band network connection. Communication may be either one-way or two-way. A multipoint communication using a separate Multipoint Control Unit (MCU) among more than two terminals is also possible. Furthermore, the multimedia telephone terminals can be integrated into PCs or workstations, or can be stand-alone units.
Interworking with visual telephone systems on mobile radio networks is defined in the ITU-T video/multimedia recommendation H.324/M. The logical unit of information exchange between a multiplex layer and an underlying physical layer is the Multiplex-Protocol Data Unit (MUX-PDU). The MUX-PDU is a packet framed by High-Level Data Link Control (HDLC) flags and using a HDLC zero-bit insertion for transparency.
According to the above video/multimedia recommendation, three levels of error correction measures or schemes are provided in order to improve the robustness, i.e. error resilience, of the multiplexed audio video or data frame, wherein level 3 provides the best error resilience. The three levels are defined as follows:    Level 1: the HDLC flag is replaced by a longer flag, HDLC bit stuffing is not used;    Level 2: the MUX-PDU header contains an error protection (in addition to the level 1 measures);    Level 3: the PDU content is error-protected (in addition to the level 2 measures).
The details of the above measures corresponding to the levels 1 to 3 are described in annexes A, B and C, respectively, of the ITU-T Recommendation H.223.
The used level is negotiated in-band between the video terminals. If either of the video terminals does not support any of the above levels, a default level is used, defined as level 0.
However, current terminals often do not support the above defined error correction schemes. Moreover, in future, there may be new error correction schemes to further improve the error resilience of the video/multimedia frames in mobile environments. This means that there will in practice be a spectrum of video terminals with various levels (from level 0 to the highest level) of error correction.
Furthermore, a huge spectrum of specifications is used in fixed networks for defining multimedia or video calls with various protocols, transfer capabilities and ways of signaling, e.g. ITU-T H series recommendations and V series recommendations.
In fixed networks, terminals set up a call and negotiate on parameters with inband procedures defined in V.8, V.8bis and V.140.
Thus, flexible interworking is required between different networks, such as mobile and a fixed network, in order to support various services.